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MG MGB Technical - Headlights dim then brighten?

I have a 1974 MGB GT which has sealed halogen headlights. I have noticed that when the headlights are on low beam, they illuminate fine, then all of a sudden they get much brighter (as though they automatically switched to halogen). The dash lights also appear to get brighter and as they are original, I assume they are not halogen. The lights stay bright for 10 to 20 seconds then go back to their original glow. They are not switching to high beam, as the high beam indicator light works well and when the high beams are on, the headlights are much brighter. Any suggestions as to why this would happen?
Tim

Tim,
Sounds as though your voltage regulator could be acting up, maybe monemtarily supplying too high a voltage, then reverting back to proper operation. This would explain the temporary increase in brightness. Just a wild guess, but seems logical to me.

Joe
Joe Ullman

Is this happening at road speed or only at idle? Is the fan belt in good condition and tight? Have you checked the grounds to make sure they are not loose? You should have an internal voltage regulator. If necessary, you could connect a volt meter to the battery clamps and run the lines forward to where you can observe them. If you see a voltage surge (above 13.5 to 14.5) volts, you would then have an indication that the voltage regulator was allowing excessive voltage into the system. Normal charging voltage is 13.5 to 14.5 volts. More than this indicates a problem. Joe may be right about a power surge, but most of my experience is with a loss of voltage with the alternator output showing less than 13.5 volts when the regulator begins to fail. If your fanbelt is good, properly tensioned and you have good grounds, hook up a meter and see what is happening. Les
Les Bengtson

Thanks guys for your comments.

To answer some of your questions...this happens at road speed for no apparent reason (not hitting bumps, high rev's, nor at any particular time such as the beginning or end of a run). I have not noticed this happening at idle nor have I noticed it with the bright lights on. The fan belt is brand new and tight. The alternator was taken to a shop in the spring and rebuilt, therefore as it would have an internal voltage regulator should this not have been checked as well? I have not checked the grounds...as to be honest, I'm not positive where they are located. Can you give me some advise as to where to look for the grounds for this system? Also, if the ground was poor, wouldn't the lights go off, not get brighter? I must admit...I'm a lot more confident in my mechanical skills as opposed to my electrical. Thanks for your help.

Tim
Tim

Basic system grounds are at the rear of the car outside the battery box and a ground strap from the engine mounting on the left side of the engine to the motor mount on the chassis below it. Those could be faulty, causing a general system fault.

However, if only the headlamps are affected, then my guess would be that the load presented by the lamps is sufficient to cause some weak component within the alternator sub system to act up. Could be intermittent from heating up due to high current demand, or an internal weakness of a solid state or regulator component, or even the breakdown of the insulation somewhere in the alternator/regulator assembly, or even a crappy wiring connection.

One connection sometimes overlooked is the one where the heavy Brown wire from the fuse box is connected to the solenoid battery post. If this connection is cruddy or loose, you can get some really high resistances from time to time, depending on the load on the system. Also, check your battery cable and its connections to make sure they are in good condition and mechanically/electrically sound.

Also, don't discount the fuse block's ability to contribute to these same symptoms. An old and cruddy fuseblock can present some real troubles. If yours is more than 15 yrs old, it couldn't hurt to replace it. And if you do, get some rosin core solder and solder all the riveted connections on the Back side of the fuse block as it is these connections which corrode over time and present the real problems most folks encounter from this component. Soldering them will make for much better connectivity and joint integrity over time. FYI, and good luck.

Let us know what you find is the real problem so we can learn something too!! ;-)
Bob Muenchausen

My '79 B has the same problem except it only brightens for a second or two,I haven't had a chance
to check everything but I replaced my headlights with halogens too.could it be the lights?Are they pulling too much current for the system?I'm going to go back to regular lights and see if that makes a difference
I'll let the list know what I find.
Pat
Pat Bailey

Question is, which is correct - dim or bright? You need to check the system voltage. It should be about 14v at a fast idle or higher. A dodgy ground around the battery could be the cause, but would likely cause cranking problms too. A dodgy brown around the alt could also cause it, but would likely cause charging problems. I've never know halogen bulbs on their own be the cause of intermittent brightening and dimming. Sounds more like alt regulator problems than anything else to me.

PaulH.
Paul Hunt

Thanks a lot guys...you've been a real help.

I will start by checking grounds then use a volt meter to see what's happening. Perhaps the regulator has gone bad on me!

Tim
Tim

This thread was discussed between 12/09/2001 and 18/09/2001

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